This present disclosure relates generally to systems and methods for solid or biological slurry filtration, harvesting and cultivation, and more specifically to algae harvesting and cultivation systems and methods.
It has long been recognized that algae harvesting is a major deterrent to realizing practical and economical unicellular algae production. Algae is typically cultivated at 0.02% to 0.5% solid concentration, so large amounts of water must be removed from algae mediums to recover algae product having a high algae concentration (e.g., 3% to 20% solids content). Commercial algae harvesting facilities typically use a centrifuge or a dissolved air floatation system followed by centrifugation to harvest and dewater algae. Centrifuges, however, have high capital and operating costs, and dissolved air floatation systems typically require an addition of a coagulant or flocculent, which increases operating costs. Electrocoagulation, cross flow filtration, bioflocculation, vibrating membrane filtration and ultrasonic harvesting have been proposed as alternatives to centrifuges and air flotation systems, but an algae harvesting system having low operating costs and minimal energy requirements has remained elusive.
One approach to general solid separation has been outside-in hollow-fiber dead-end filtration in an atmospheric pressure system. These systems include multiple porous hollow fibers, which can be grouped or arranged into modules. The modules can be grouped into cassettes having multiple modules, and the cassettes can be grouped into banks of multiple cassettes. The hollow fibers are immersed in a liquid suspension, and filtrate or permeate can be drawn through walls of the fibers and out of the fiber lumens. A concentrate or retentate with the retained solids remains outside of the hollow fibers. The fibers can be arranged vertically, horizontally, or at an intermediate angle in the liquid suspension. In large hollow-fiber dead-end filtration systems, modules are typically contained in concrete basins or tanks made of metal or plastic to minimize the amount of extra fluid in the system, attain higher concentrations of solids, and reduce the amount of fluid required for membrane washing and cleaning. For large filtration systems, very high volumetric flows are used, resulting in high costs for concrete basins or tanks to contain the hollow-fiber membranes.
Membrane fouling is a significant problem with these hollow fiber dead-end filtration systems. In general, membrane fouling occurs when a solution or particle gets deposited on a surface or in the pores of a membrane causing the membrane's filtration performance to be degraded. Typical methods to reduce membrane fouling with hollow fiber membranes include introduction of air bubbles around the hollow fibers of the membrane, moving the hollow fibers within the liquid suspension, periodic backwashing (also called back-pulsing or backflushing), periodic chemical cleaning, and periodic draining of the liquid suspension. Backwashing is a process in which a fluid is forced through the fibers of the modules typically at a flow rate that is greater than the rate at which permeate is withdrawn. Fibers may be backwashed with a liquid such as water, or a gas (e.g., air) or a mixture of gas and liquid. When water or a liquid permeate is used for the backwashing, the backwash is essentially a recycling process in which the solids production rate is sacrificed during the backwash and during the time it takes to re-filter the water or permeate that was used for the backwash. A water or permeate based backwash system is therefore justified primarily when the cleaning effect is significant. In hollow-fiber dead filtration at atmospheric pressure, the maximum delta or change in pressure for backwashing or permeate flow is typically about eight pounds per square inch (psi), so controls are needed on the pumps to prevent over-pressurizing the membranes and to control the variation in pressure when the permeate and backwash valves are opened and closed.
Periodic backwashing is typically utilized several times per hour in solid filtration systems, e.g. backwash intervals of 15-30 minutes. The backwash offline period is typically 30-120 second and can include the time to open and close valves, the time for the backwash fluid to flow, and the time for any pulsing or adjusting of any pump or compressor during the backwash flow. When water or permeate is used for the backwash, the backwash process is essentially a recycling process in which the solid production rate is sacrificed during the backwash off-line period and during the time to re-filter the water or the permeate that was used in the backwash. Backwashing is therefore justified to the extent that the cleaning effect is significant. Attempts to optimize backwashing in hollow fiber dead-end filtration systems have indicated that as suspended solids concentration is increased, the backwash off-line period is typically increased to allow for a longer time for backwash flow.
Pumps are typically used to provide a permeate or liquid backwash. Systems utilizing pumps, however, can be very complex and costly, and often utilize variable frequency drive (“VFD”) pumps. For these systems to work without over-pressure, multiple valves typically need to be open and closed virtually simultaneously. Air pressure has been proposed as an alternative to liquid backwash, but the cost of pressurizing air is much greater than liquid, and introducing air into permeate channels can cause problems.
Unlike ceramic or metallic filtering membranes, the backwash pressure in hollow fiber membranes is limited to avoid damaging the fiber membranes. The backwash pressure used in hollow fiber membrane systems is typically well below the maximum to avoid membrane damage from spikes or transients when the backwash is started and stopped. Complicated controls are required to minimize these transients and pressure spikes. Furthermore, the low-pressure tolerance of hollow fibers prevents the use of short, high-pressure back-pulses that are used in ceramic or metallic membrane systems to remove fouling by a pressure shock.
Biological slurries such as algae or activated sludge are typically more difficult to filter to high suspended solids concentrations than inorganic slurries. Natural or synthetic flocculants are typically required to attain greater than 1% suspended solids. The addition of flocculent, however, is costly and can negatively impact the processing or value of the algae product. Activated sludge is a consortium of microbes in which natural bioflocculation is attained, so hollow fiber dead-end filtration can be used for activated sludge. However, the maximum concentration of suspended solids with naturally flocculated activated sludge is typically about 3% to 4.5% when dead-end hollow fiber filtration is used for activated sludge.
Non-flocculent, cross-flow membrane filtration systems have been used in an attempt to attain a high concentration of algae product. Cross-flow filtration systems, however, have higher energy requirements and higher operating costs than dead-end filtration systems. For example, typical cross-flow filtration systems can require 0.4 to 7 kWh/m3 of energy to operate. Cross-flow filtration systems are therefore less economical than dead-end hollow fiber systems. In addition, cross-flow systems have higher shear stress and have recirculation in the cross flow pump loop, which can damage algae cells.
Most hollow-fiber liquid filtration systems are single stage. Multistage hollow fiber solid filtrations systems have been used with constant flux in each stage to achieve higher average flux. These constant flux and constant area multistage systems typically produce low solid concentration (e.g., less than 1% suspended solids), and require active transmembrane pressure control and active fluid flowrate control for each stage, which increases the cost and complexity of such systems.
Production of algal products is often enhanced by two-stage cultivation in which algae is pretreated before entering a second stage or the algae media is altered in the second stage. In some cases, stress from media changes, such as nitrogen deprivation, salinity, or pH is used to induce formation of a product. In other cases, exposure to stress such as shear, ozone, bleach, or high light is used to induce formation of a product. If the media is changed, then recovery and recycle of the media for cultivation is prevented because salts or other dissolved solids are added to the media, and high operating costs are incurred because chemicals must be added to each batch to modify the media. If exposure to stress is used, then the amount of chemicals or size of the second stage pretreatment system is large because the algae are cultivated under dilute conditions.
Aquaculture facilities often require live feeds to feed fish, shellfish, and larva of fish or shellfish. These algae could be produced more economically in centralized facilities, but shipment of dilute cultures is expensive, and algae harvesting processes damage the algae or require flocculants, so concentrated algae cultures are not available. Dead algae products are centrally produced and shipped for use in aquaculture facilities, but these products are not as effective as live algae. Thus, typical aquaculture facilities must cultivate algae for feed in addition to cultivating fish or shellfish.
Concentrated algae slurries attained in harvesting and dewatering contain extra-cellular media, and the algae slurry is often dried to obtain an algae product. The dissolved solids in the extra-cellular media increase the ash content of the dried algae product and can add undesirable compounds such as metal salts to the product. In some cases, algae slurries are processed to lyse the cells or extract a product. In many of these processes, the lysis or extraction is more effective with a particular ionic composition, pH, or osmotic strength in the extra cellular media. Adjustment of the media is difficult because it typically involves re-suspension in a new media followed by another expensive and energy-intensive harvesting step.
In view of the above, it should be appreciated that new and improved algae harvesting and cultivation systems and methods are needed.